Optical fibers are fabricated by heating and softening the end of a glass rod form of intermediate products called preforms, and drawing the same. At this time, the internal refractive index structure required for an optical fiber is formed on the preform stage, and the performance of the optical fiber to be fabricated is determined by whether this structure is acceptable or not. Of importance to this end is precise control of refractive index profiles at an optical fiber fabrication step, and measurement for checking the same. As the precision of the measuring instrument used for this measurement becomes worse, the fabricating yield of optical fibers becomes low and the cost of optical fibers is adversely affected.
The determination of the internal structure of an optical fiber perform has so far been made using the bending of light rays traversing the interior of the perform, as set forth in I. Sasaki, D. N. Payne and M. J. Adams: “Measurement of refractive index profiles in optical fiber performs by spatial filtering technique”, Electronics Letters, 16, 6, pp. 219-221.1980-03.
When parallel rays are used, their bending is measured upon traversing the preform, and with a method making use of image distortions (U.S. Pat. No. 3,072,986), the bending or inflection is determined from the angle of incidence. The latter has a feature of being more compatible with a wider range of performs as compared with the former.
In the latter method, an image is located behind a columnar form of optical fiber preform having an uneven refracting index distribution therein. Used for the image is a pattern wherein a light ray starting from one point on an image carries information on positions within that image. A picture of that image is taken across the optical fiber preform. A distortion of the thus taken image is analyzed to find a deflection function, on the basis of which the refractive index distribution of the optical fiber perform is determined.
For an actual measuring technology, however, an oblique edge image is utilized. A problem with this technology is that it is not easy to precisely measure the refractive index distribution at the plane vertical to the axis of the preform. It follows that there is a problem in connection with a preform having a refractive index profile fluctuating in the axial direction.